The invention presented herein relates to clocking circuitry for providing clocking signals in accordance with a preprogrammed sequence of rates and, in particular, to such circuitry wherein the preprogrammed sequence of rates is based in part on the known repetitive movement of a mirror to which the output of a laser is directed.
Laser printers produce latent electrostatic images on a photoconductor by directing light from a laser to the photoconductor that has received a uniform electrical charge. The light from the laser is directed to the photoconductor in a scanning fashion with the laser turned on and off according to image defining data signals. It is important that the laser be controlled for on or off operation at the same point in one scan line as in the preceding scan line. If this action is not precisely controlled, images will appear irregular and portions of an image intended to present vertical lines will not be exactly vertical. Arrangements are used wherein light from the laser is directed to a moving mirror which reflects the light from the laser to the photoconductor to establish a scan line. The photoconductor is adapted for movement transverse to the scan lines so the light is directed to a different portion of the photoconductor for each scan line. A self resonant scanning mirror is a desirable device for providing a moving mirror for the scanning portion of a laser printer since its movement is highly reproducible from one line scan to the next due to its high "Q". "Q" is the ratio of energy stored in the mirror device over a time interval to the energy supplied to the mirror device during such time interval. The inherent stability of the self resonant scanning mirror can be utilized in a scanning system for a laser printer provided line-to-line synchronization of data signals provided to the laser is done precisely.
While the movement provided for the mirror of a resonant scanning mirror is highly reproducible, the velocity of movement is sinusoidal, which requires the clocking of image defining data signals for operation of the laser to be matched to the sinusoidal velocity of the mirror if there is to be uniform spacing of the image areas in a scan line. Prior approaches for providing a solution to this problem include the use of a second light beam plus a ruling or grating to generate clock signals. This is a cumbersome and expensive approach. Another approach, which also fails to provide the accuracy desired, uses the tachometer output of the scanning mirror to regulate the rate of clocking. Such an approach requires a solution to the inherent electrical noise component and microphonic noise component that is present in the tachometer output.